@Ironclad: Be fair, even by her own admission in the previous chapter she handled the revelation quite poorly by simply opting to abruptly force a distance between the two of them instead of actually discussing the matter with her. Plus, for all the emotional maturity that Marie has exhibited in comparison to Mel and Fran, she's still has a childlike mentality just like the two other Beastkin, and furthermore she's in love with Mel, thus it's totally expectable for her to take Master's actions very badly and be very biased against her rather than try to look at the situation from Master's side. Stella, for her part, admits that she doesn't know the details, but is erring on the side of caution by fulfilling Marie's favor (i.e. that Marie may have a very good reason to want to keep Master away from Mel) and giving a very important aesop to both Master **and** the audience: Even if the society around them treats Beastkin as if they have no say in the matter, masters like Stella and Master are supposed to be better than that and respect their Beastkin companions' right to self-determination as fellow sapient beings (if not fellow "humans"/people).
And yes, I know Master probably didn't consciously mean it that way when she tried to assert her legal rights as Mel's Master to force her way into seeing and talking with Mel, but it's important to nip such things in the bud before they take root into a person's heart.